Hygroscopicity of secondary marine organic aerosols: Mixtures of alkylammonium salts and inorganic components

Abstract

Field studies have identified alkylammonium salts as important components of secondary marine organic aerosols. In this work, we study the hygroscopic behavior of laboratory-generated alkylammonium aerosol particles, including monomethylammonium chloride (MMACl), dimethylammonium chloride (DMACl), trimethylammonium chloride (TMACl), diethylammonium chloride (DEACl), and their mixtures with inorganic salts containing ammonium sulfate (NH4)2SO4, sodium chloride NaCl, calcium nitrate Ca(NO3)2 and sodium sulfate Na2SO4 at different dry mass ratios with a hygroscopicity tandem differential mobility analyzer (HTDMA). The hygroscopic growth of pure alkylammonium salt particles (except for DEACl) reveals gradual water uptake over the whole studied range of relative humidities (RHs). In general, the impact of the presence of alkylammonium chloride on the phase behavior and hygroscopic growth of mixtures depends on the chemical composition of particles and volume fraction of the alkylammonium chloride in the mixtures. For alkylammonium/(NH4)2SO4 mixed particles (except for TMACl/(NH4)2SO4), the hygroscopic growth shows a smooth growth tendency when the organic content is high, while the deliquescence transition is observed for alkylammonium salt/NaCl mixtures at all mass ratios. Regarding the different mixtures of alkylammonium/Ca(NO3)2 particles, continuous water uptake without phase transition is observed over the studied RH range, indicating that alkylammonium salts impose no effect on the liquid-like state of calcium nitrate. The alkylammonium/Na2SO4 mixtures show obvious particle shrinkage prior to the deliquescence point. A similar behavior is also observed for alkylammonium salt/NaCl mixtures. The observed diameter reduction can be attributed to the transformation of porous or irregularly shaped solid particles into more compact near-spherical particles. In the following, measured growth factors (GFs) are compared with values predicted with the Zdanovskii–Stokes–Robinson (ZSR) mixing rule and ideal solution model. The ZSR predictions for different alkylammonium/inorganic mixtures are similar to the measured GFs as long as the mixed particles are in a liquid-like state.